Coated carrier particles with magnitude of triboelectric charge controlled and method of making same

ABSTRACT

AN ELECTROPHOTOGRAPHIC DEVELOPMENT CARRIER COMPRISING CORE PARTICLES COATED WITH A CARBOXYLATED POLYETHYLENE IONOMER. WHEN UTILIZED WITH APPROPRIATE ELECTROSCOPIC TONER MATERIALS, AND CARRIER, WITH IS COATED WITH A CARBOXYLATED POLYETHYLENE IONOMER IS EFFECTIVE IN PRODUCING A DESIRED POLARITY AND MAGNITUDE OF THE TRIBOELECTRIC CHARGE ON THE TONER PARTICLES. CONTROL OF THE POLARITY AND MAGNITUDE OF THE TRIBOELECTRIC CHARGE CHARACTERISTIC OF THE CARRIER, WHICH HAS ITS PARTICLES COATED WITH THE CARBOXYLATED POLYETHYLENE IONOMER IS OBTAINED BY CONTROLLING THE CURING TEMPERATURE OF THE CARBOXYLATED POLYETHLENE IONOMER.

United States Patent 3,725,118 COATED CARRIER PARTICLES WITH MAGNI- TUDE OF TRIBOELECTRIC CHARGE CON- TROLLED AND METHOD OF MAKING SAME Sterritt R. Fuller, Jr., and Howard E. Munzel, Lexington, and Kenneth D. Woosley, Winchester, Ky., assignors to International Business Machines Corporation, Armonk, N.Y. No Drawing. Filed June 23, 1971, Ser. No. 156,060 Int. Cl. G03g 13/08; B32b 15/08 US. Cl. 117-100 M 5 Claims ABSTRACT OF THE DISCLOSURE An electrophotographic development carrier comprising core particles coated with a carboxylated polyethylene ionomer. When utilized with appropriate electroscopic toner materials, the carrier, which is coated with a carboxylated polyethylene ionomer is effective in producing a desired polarity and magnitude of the triboelectric charge on the toner ptrticles. Control of the polarity and magnitude of the triboelectric charge characteristic of the carrier, which has its particles coated with the carboxylated polyethylene ionomer is obtained by controlling the curing temperature of the carboxylated polyethylene ionomer.

In electrophotography, a photoconductor is charged and then exposed imagewise to light. In the area of the photoconductor exposed to light, the charge dissipates or decays whilethe dark areas retain the electrostatic charge.

The difference in the charges between the areas exposed to light and the dark areas produces electrical fields therebetween. Accordingly, the resultant latent electrostatic image on the photoconductor is developed by depositing small colored particles, which are known as toner particles, over the surface of the photoconductor with the toner particles having a charge so as to be directed by the electrical fields to the image areas of the photoconductor to develop the electrostatic image.

A number of means are known for developing the latent electrostatic image by the application of the toner particles. One of these is known as cascade development. Another means is known as the magnetic brush process.

In each of the cascade and magnetic brush development processes, a two-component developer material is utilized. The developer material comprises a mixture of small toner particles and relatively large carrier particles. The toner particles are held on the surfaces of the relatively large carrier particles by electrostatic forces, which develop from the contact between the toner and carrier particles producing triboelectric charging of the toner and the carrier to opposite polarities. These polarities are determined by the relative positions of the materials in the triboelectric series. When the developer material is moved into contact with the latent electrostatic image of the photoconductor, some of the toner particles are detached from the carrier particles by a combination of mechanical forces and electrostatic forces and are attracted to the latent image.

The toner and carrier particles of the developer material are specially made and processed so that the toner obtains the correct charge polarity and magnitude of charge to insure that the toner particles are preferentially attracted to the desired image areas of the photoconductor. For a given developer-hardware system, the magnitude of the triboelectric charge is important in that if such charge is too low, the copy will be characterized by high print density but heavy background; if the charge is too high, the background is good but the print density will tend to be low. Thus, there is an optimum range of toner charge for best overall results.

Prior art carriers have been particularly sensitive to toner filming, with the exception of carriers such as those disclosed in the copending application of C. A. Queener et al., entitled Coated Carrier Particles, Method of Making Same, and Improved Electrophotographic Process, Ser. No. 110,756, filed Ian. 28, 1971, and application of William J. Kukla et al., entitled Coated Carrier Particles With Polarity and/ or Magnitude of Triboelectric Charge Controlled, Method of Making Same, and Improved Electrophotographic Process, Ser. No. 110,725, filed Jan. 28, 1971.

The carrier disclosed herein exhibits some of the characteristics of the carriers disclosed in the above applications while possessing better toner filming resistance than prior art carriers other than those above. There are three primary modes of carrier failure: abrasion, Spalling, and toner filming. Spalling occurs when the bond between the coating and the core breaks and coating material breaks up and separates from the core.

Toner filming is the phenomena of toner particles fusing to the carrier coating and thus reducing the capacity of the coating to triboelectrically charge the toner particles.

Because of the contract between the carrier particles and between the carrier particles and parts of the machine, there is abrasion of the coating of the carrier particles. This abrasion of the coating also may reduce the effectiveness of the triboelectric charging between the carrier and the toner by exposing the toner to the core material of the carrier.

Thus, if the coating is not sufiiciently resistant to abrasion, early replacement of the entire developer material is required. This replacement of the entire developer material is costly and time consuming, especially in high speed copy machines.

Furthermore, even if the coating of the carrier particle resists abrasion, the coating also must have good adhesion to the core of the carrier particle. Otherwise, the coating can chip, flake, or spall, even if the coating is of a material that is not subject to abrasion, due to the rubbing or contact between the various carrier particles and between the carrier particles and parts of the machine. This also requires early replacement of the developer material.

Therefore, in addition to having the proper triboelectric characteristics, the coating of a carrier particle must have good anti-stick (low surface energy) properties to prevent filming of the carrier particle by the toner. The coating also must have good adherence to the core and be resistant to abrasion.

In addition to having the foregoing desired properties, the carrier particles must have the characteristic of having a triboelectric charge of a desired magnitude and polarity when used with a particular el-ectroscopic toner. This is because the magnitude of the triboelectric change of the toner is controlled by the magnitude and polarity of the carrier charge when the toner and the carrier are mixed.

The magnitude of the charge of the toner is important for the electrophotographic system to produce copies of a high quality. If the magnitude of the charge of the toner is too low, the copies have poor edge definition and lack contrast since the non-image or background areas possess an unacceptable level of toner as previously mentioned. If the magnitude of the charge of the toner is too high, then there is poor image fill since the toner would tend to stay with the carrier. Accordingly, the magnitude of the charge of the toner must be within a range above that in which poor edge definition is produced and below that in which poor image fill occurs.

Thus, if the coating of the carrier particle has the characteristic of impartingatriboelectric charge to the toner when mixed therewith so that the toner charge does not fall within the range in which copies of high quality can be obtained, the coating cannot be employed for a carrier for the toner even though it meets the abrasion, anti-stick, and adhesion requirements. Accordingly, for a carrier coating to be satisfactory, it not only must meet the abrasion, anti-stick, and adhesion requirements but also must be capable of causing the carrier to have the characteristic of triboelectrically charging the toner when mixed with the toner so that the charge of the toner is within the desired range that enables copies of high quality to be produced.

From the location of polyethylene in various published triboelectric series, and experimentation with polyethylene, one skilled in the art is led to reject it as a carrier coating even if it could meet the other requirements, because the charge magnitude capability of the polyethylene is so low as to provide degraded copy qualities.

The present invention has overcome the above problems by using a carboxylated ionomer of polyethylene and then curing it under controlled conditions. The carboxylation makes the polyethylene generally more electronegative and moves its position in the triboelectric series toward the negative end. By controlling the curing conditions of the carboxylated polyethylene ionomer coated carrier, the magnitude of the charge characteristic may be controlled. Also, if desired, by controlling the curing conditions, the carrier coating may be forced to a more electropositive position in a triboelectric series. Thus, by controlling the curing condition and the extent of carboxylation of the polyethylene, it is possible to select a toner and then match the carrier to it rather than having to develop a toner and carrier simultaneously and in parallel as a developer composition.

Accordingly, even if the potential of the image areas in an electrophotographic system were changed significantly so that the charge and/or polarity of the toner must be changed for the toner to continue to be employed in the electrophotographic system, this invention allows the particular toner to still be used. Additionally, this invention permits the selection of any one of a plurality of toners for use with a particular electrophotographic system.

At the same time that this flexibility of the toner charge through controlling the triboelectric characteristic of the carrier material is available by employing the method of the present invention, the present invention also insures that the carrier particles have a relatively long life. This is because the coating of the carrier particles has the required anti-stick, abrasion, and adhesion properties necessary for a long life coating for a carrier.

An object of this invention is to provide a new coated carrier particle having the magnitude and/ or polarity of its triboelectric charge controlled in accordance with the toner and the electrophotographic system with which it is used.

Another object of this invention is to provide a new electrophotographic development carrier having a relatively long life.

Still another object of this invention is to provide an improved electrophotographic process.

The foregoing and other objects, features, and advantages of the invention will be more apparent from the following more particular description of the preferred embodiments of the invention.

The core of the carrier particle or bead formed by the present invention may be any material to which the coating can adhere and which can withstand the curing temperature. Thus, the material of the core of the carrier particle may be sand, glass beads, metallic beads, or metal alloys, for example, or any of the many accepted carrier particle materials.

When the carrier particles of the present invention are employed in a developer utilized in the magnetic brush process, the material of the core is a ferromagnetic ma terial such as iron or steel, for example. Other suitable ferromagnetic materials such as magnetic oxides and alloys (copper-nickel-iron for example, may be employed.

The size of the core generally may be bet-ween 50 and 1,000 microns. The preferred size range is between and 600 microns.

The material of the coating of the core of the carrier particle of the present invention is formed of a carboxylated polyethylene ionomer. Carboxylated polyethylene ionomers are sold by E. I. du Pont Corporation under the trademark Elvax D which are dispersions in water of ionomer resins.

In applying the coating to the core, the coating may be applied by any suitable means such as dipping, spraying, tumbling the cores with a coating dispersion in a barrel, or through a controlled fluidized bed. The fluidized bed process is preferred since it enables a uniform coating to be applied to the cores of the particles. The fluidized bed process is well gnown in the prior art as a coating technique.

In a fluidized bed process, for example, the cores are suspended and circulated in an upwardly flowing stream of heated gas such as air, for example, in a manner such that the particles move upwardly and are sprayed by the coating material in a first zone. Then, in a second zone, the particles settle through the air stream in a zone of lower air velocity. The liquid, which is a solvent and/or a dispersant, of the sprayed coating evaporates to leave a thin solid coating on the particles. The particles recirculate to the first zone so that successive layers of the coating material are built up on the core in a uniform manner.

After the core has been coated to form the carrier particle, the coating is cured so as to possess the desired triboelectric properties. The curing process depends upon the material of the coating and the composition of the toner with which the carrier particles of the present invention are to be employed, as well as the triboelectric requirements of the system in which it is used.

Thus, if the coating is a carboxylated polyethylene ionomer, for example, then curing of the coating occurs by heating the carrier particles at a selected temperature for the selected period of time; the carrier particles are then allowed to cool to room temperature, thereby afiecting the magnitude of the triboelectric charge of the carrier particle when mixed with a particular toner. Thus, for a particular toner, the carboxylation of the polyethylene and the curing produces substantial increase in the magnitude of the triboelectric charge over the uncarboxylated polyethylene. This enables the triboelectric charge of the toner to fall within a desired range in which high quality copies are produced.

It should be understood that the curing temperature must be sufiiciently high so that the coating will soften or melt and adhere to the core. The desired magnitude and polarity of the toner charge for the particular toner determines the actual temperature selected.

The thickness of the coating may vary from less than 1 to about 20 microns. However, it is preferably about 1 to 5 microns. The coating thickness must be suflicient to impart the desired triboelectric effect to the carrier with the upper limit of the thickness being determined by physical integrity of the coating.

The following are examples of the present invention in detail. The examples are included merely to aid in the understanding of the invention, and variations may be made by one skilled in the art without departing from the spirit and scope of this invention.

The particles or the core materials in practice are selected for size and material and placed within a coating apparatus which may, for example, be a fluidized bed coating apparatus or other apparatus Well known in the art or coated by tumbling, spraying, dipping, or other recognized techniques. For the sake of illustration, the following examples will utilize the fluidized bed technique of coating the core materials.

The core particles are placed in a chamber, the chamber being such that a spray of the material to be coated is injected into an upward moving air stream and is carried by the air stream into contact with the core materials. The spray of material is injected in the center of the air stream such that the materials are carried up with the air stream to an equilibrium height and then fall back through the air flow to a lower height where they re-enter the upward air flow where it is again combined with the coating material and receive subsequent coats until the desired coating thickness is obtained. The following examples will more clearly illustrate the characteristics and preparations of a carrier material made from a core material and a form of carboxylated polyethylene ionomer.

EXAMPLE 1 16 pounds of 8-70 spherical steel shot were loaded into a fluidized bed coating apparatus. A water dispersion of carboxylated polyethylene ionomer designated by E. I. du Pont as Elvax D-1236 was diluted with two parts of deionized water to one part of the Elvax D-1236 dispersion such that the solid percentage was approximately 14 to 16 percent to yield 600 ml. of dilute dispersion. Air was then forced under pressure into the fluidized coating apparatus at 150 to 1170 F. and the resulting dispersion is pumped through an atomizing spray nozzle at a rate such that uniform coating occurs and is slow enough to allow minute quantities to coat the steel shot and partially dry prior to the steel shot being caught in the fluidized bed for a second pass of coating. This repetitive coating of a steel shot continues until all shot in the batch are coated to the desired thickness size which approximates 3 to 3 /2 microns of coating thickness. The approximate rate that the diluted dispersion is atomized and injected into the air fiow is ml. per minute. A total volume of coating material after dilution is approximately 600 ml. for a 16 pound batch of 8-70 spherical steel shot. After the coating was completed, the steel shot coated with Elvax D-l236 is then circulated in a heated air current for approximately minutes at a temperature of approximately 150 to 170 F. inlet temperature. This drying step drives off most of the remaining water and leaves a batch of coated steel shot which is essentially dry and not agglomerated.

As was discussed above, the triboelectric charging capability and polarity of the carrier is partially dependent upon the cure temperature and cure time. The shot, coated as above, was then cured for approximately minutes at a temperature of approximately 200 F. This additional curing after coating dries off residual liquid material, in this case, de-ionized water, and further cures the carboxylated polyethylene ionomer to acquire desired triboelectric charge capabilities. The curing, as described above, in addition to driving off the remaining carrier fluid, aids in acquiring the best possible bond between the coated material and the steel shot. Since the temperature to which it is heated during the cure period is above the glass transition temperature of the coating, any surface irregularities, roughness, or particle of carboxylated polyethylene ionomer whch have heretofore not been thoroughly melted and/or softened to allow a uniform spherical coating are then heated to a temperature which allows the coating to become a uniform thickness, smooth surface, and spherical.

The resulting carrier was then tested to determine its triboelectric characteristics and its capabilities for making acceptable photocopies and the results of these tests indicated that triboelectric characteristics of the carboxylated polyethylene ionomer marketed under the trade name of Elvax D4236, when coated and cured as above, produced good copies. The test copies had excellent background, i.e., low background, and excellent line definition.

The test copies were made utilizing the magnetic brush development technique wherein a magnetic brush attracts the steel core of the carrier beads and carries them, together with the triboelectrically charged toner particles onto the surface of the photoconductor, thus toning and 6 developing the latent electrostatic image carried by the photoconductor.

EXAMPLE 2 A water dispersion of a carboxylated polyethylene ionomer marketed by Du Pont under the trade name Elvax D-1070 was prepared by diluting the dispersion in proportions of one part dispersion to two parts de-ionized water thereby reducing. the minimum solids to the approximate range of 14 to 16%, and it was coated upon S70 shot in the same quantities as in Example 1. The conditions of the coating process were the same as described in Example 1 as was the curing procedure.

The resulting carrier was then tested for its triboelectric characteristics and indications were that the triboelectric charge capabilities of this carrier were good to excellent.

EXAMPLE 3 -In this example, a water dispersion Elvax D-1220 carboxylated polyethylene ionomer having a cross linkable capability was diluted two parts deionized water to one part dispersion of Elvax D-1220. This material was applied in the same manner as that described in Example 1 to a total of 14 pounds of S 70 steel shot. The total quantity of diluted dispersion used in coating the 14 pounds of shot was 450 ml. The input temperature of the fluidized coating chamber was to F. and the coating rate was approximately 15 ml. per minute. After the coating was completed, the coated core material was then dried in a heated air flow for approximately 10 minutes at 115 to 120 F. The carrier particles were then removed from the fluidized bed coating apparatus and oven cured at a temperature of approximately 250 F. for a period of approximately 20 minutes. The resulting material was broken up and screened to result in relatively uniform carrier bead material. The resulting carrier beads were then tested for triboelectric charge capability. The resulting carrier beads were then used in the production of electrostatic copies. The copies were excellent having excellent line and half-tone qualities and some low background.

EXAMPLE 4 The same general process as described in Example 1 was performed using Elvax D-1249 carboxylated polyethylene ionomer having a very high molecular weight and having been diluted two parts de-ionized water to one part of carboxylated polyethylene ionomer dispersion. This mixture was then introduced into a fluidized bed coating apparatus where a total volume of 800ml. of the above dispersion was coated onto 24 pounds of 8-70 spherical steel shot. The coating was performed in an air stream having an inlet temperature of to 190 F. at a coating rate of 30 ml. per minute. When coating was completed, the shot was continued to be circulated in the fluidized bed coating apparatus for a period of 30 minutes at a temperature of to 200 F. thereby drying and curing the carrier beads. The combined curing and drying performs the same function as a drying in the fluidized bed apparatus and subsequent oven curing or a subsequent curing in the coating apparatus. The curing of the carrier particles in the fluidized bed apparatus prevents agglomeration of the particles and obviates the need to break up an agglomerated mass such as is formed by the technique of oven curing as described in Example 3. Thus, the particle coatings are uniform, smooth, and spherical with properly cured in the fluidized bed coating apparatus.

The carrier material thus formed was tested for its triboelectric charge capabilities and produced excellent copies with excellent line definition, excellent half tone qualities, and low background toning. As can be seen from the foregoing examples, it is apparent that the cure temperature is selected to be as high as possible to minimize the time required to remove the last water or carrier liquid but yet is not so high as to deteriorate the triboelectric charge capabilities or cause agglomeration within the fluidized bed apparatus.

In all of the above examples and tests, the copies were produced with a toner designated as IBM Part No. 1162013 and a magnetic brush developer. IBM Part No. 1162013 is a toner comprising carbon black pigment, polyvinyl stearate, a copolymer of styrene/n-butyl methacrylate resin, and a non-phenolic modified maleic rosin. The toner concentration in the tests referred to above was .5 to 1.0 percent concentration by weight and was used in an otherwise conventional developer system.

It should be understood that the core material in the above examples was selected as 8-70 steel shot. However, the -70 steel shot should not be considered as the only acceptable substrate material. The other acceptable types of material would be 8-110 steel shot, 250 micron worked steel grit, 450 micron worked steel grit, sand, and other well-known carrier materials. The selection of the core material will depend upon the type of developer mechanism employed with the carrier developer mix. It likewise depends on process speed and configuration of the photocopying machines. For example, if a magnetic brush developing mechanism is used, the selection of the core material must be a magnetic material. However, it should be recognized that if the developing mechanism is a cascade mechanism, sand, steel shot, worked grit, or other core material may be used so long as its density, size, and other properties are acceptable from a dynamics standpoint. One further requirement would be that any core material selected must be coatable with the carboxylated polyethylene ionomer in order to become a satisfactory carrier material.

As a preliminary to the treatment of the core material with carboxylated polyethylene ionomer, the core material may be pretreated with a primer material or other pretreatment process to improve the adhesion to the core of the carboxylated polyethylene ionomer. Examples of primers which might be used as above are polyvinyl butyral resin, vinyl chloride terpolymer, and a polymethylmethacrylate.

Pretreatments may by way of example include cleaning, pickling, etching, and roughening or other surface treatments which would enhance adhesion between the core and outer layer.

Further, pigments or other additives may be added, if desired, to the carboxylated polyethylene ionomer coating. Also, other polymers may be added to modify the triboelectric behavior of the coating as is well known in the prior art, if desired.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A carrier material comprising a plurality of beadlike particles, each having a core portion and an outer continuous coalesced surface layer wherein the improvement comprises:

said outer surface layer comprises a major portion of cured carboxylated polyethylene ionomer. 2. A carrier material of claim 1 wherein said outer surface layer comprises essentially cured carboxylated polyethylene ionomer.

3. A carrier material of claim 1 wherein said outer surface layer has a thickness of from less than about one micron to about twenty microns.

4. The carrier material of claim 1 wherein said outer surface layer has a negative triboelectric charge with respect to the toner particles with which said carrier particles are used.

5. The method of making an electrophotographic development carrier for use with toner particles comprising the steps of:

coating core materials with a thin layer of a water dispersion of carboxylated polyethylene ionomer;

heating the coated core materials to such a temperature as to partially dry said core material outer layer;

repeating said coating step and said heating step until a desired thickness of carboxylated polyethylene ionomer is deposited on said core material;

heating said coated core materials to a temperature and for a time sufficient to remove the residual carrier liquid of said dispersion and heat the core materials and core coating to a temperature above the glass transition temperature and said coating, such that said coating of said carboxylated polyethylene ionomer is coalesced into a smooth uniform film.

References Cited UNITED STATES PATENTS 3,262,806 7/1966 Gourge 25262.1 X 2,754,280 7/1956 Brown et a1. 117-161 X 3,526,533 9/1970 Jacknow et al. 117-100 X 3,355,319 11/1967 Rees 117-161 X 3,093,039 6/1963 Rheinfrank 252-62.1 3,236,639 2/1966 Tomanek 25262.1 X 3,507,686 4/1970 Hagenbach 252-62.1 X 3,586,654, 6/1971 Lerman et al. 252-621 X MURRAY KATZ, Primary Examiner D. C. KONOPACKI, Assistant Examiner US. Cl. X.R. 

